A. Introduction
The use of membranes for separation processes is well known. Certain carbon membranes are particularly useful for the separation of fluids, especially gases such as oxygen and nitrogen.
The membranes may be fabricated in various geometrical configurations, such as sheet formed membranes and hollow fibers. The membranes may, be symmetrical, asymmetrical, single-component or composite.
B. Problems With Existing Membranes
It is known that impurities including heavy, condensable hydrocarbons, i.e., C.sub.6 and greater, substantially reduce separation performance of polymeric membranes. One study, for example, reported harsh performance declines in the range of 50% reduction in CO.sub.2 /CH.sub.4 selectivity for polyimide polymeric films due to saturated concentrations of toluene or n-hexane in mixed gas feeds of CO.sub.2 /CH.sub.4. See J. Membrane Sci., 103, 73-82.
Carbon membranes have superior selectivities and productivities for many separations. However, like polymeric membranes, they also have the major problem of being vulnerable to fouling due to impurities in various hydrocarbon compounds. For example, certain hydrocarbon impurities cause selective fouling of the carbon membrane resulting in reduced selectivity.
Such impurities intolerance is reported in the scientific literature, e.g., one study showed that, for O.sub.2 /N.sub.2 separation, a carbon molecular sieve ("CMS") membrane performance deteriorated rapidly with n-hexane saturated air feed, Carbon, 32 (1427), and another study showed that, with a mixed feed gas of 50/50 H.sub.2 /CH.sub.4 with toluene vapor at low pressures (150 psia), fluxes were reduced by 15 to 20%, and the H.sub.2 /CH.sub.4 selectivity was reduced by 14%. (J. Membrane Sci., 160,179-186).
Even small amounts of such hydrocarbons can significantly impair the performance of the membrane. Impurities may be removed from the fluid to be permeated by various filtration, separation or extraction techniques. These measures may involve the use of large, expensive equipment and are often not successful.
Another deficiency of known carbon membranes is an inability to perform well with high pressure feeds. The feed, e.g., for a natural gas-CO.sub.2 separation process is typically directly from the well. It is desirable to avoid pressure loss through the purification process. Reducing the pressure to satisfy membrane pressure limits is economically disadvantageous. If pressure is reduced, expensive compressors may be required to increase the pressure of the stream for passage through an export pipeline.
C. Known Methods For Making Membranes
There are several patents describing processes for producing carbon membranes (both asymmetric hollow "filamentary" and flat sheets) and applications for various gas separations. These patents teach separations of O.sub.2 /N.sub.2, H.sub.2 /CH.sub.4, CO.sub.2 /N.sub.2, or other separations. See, e.g., U.S. Pat. No. 4,685,940 for a Separation Device, U.S. Pat. No. 5,288,304 for Composite Carbon Fluid Separation Membranes, and EP Patent No. 459,623 for Asymmetric Hollow Filamentary Carbon Membrane And Process For Producing Same, each of which is incorporated herein by reference in their entireties. None of these patents teaches CO.sub.2 /CH.sub.4 separation, especially at high pressure or in the presence of impurities.
D. Deficiencies In Known Processes For Making Membranes
The prior art references do not teach a process for CO.sub.2 --natural gas separation using a carbon membrane at high pressures and in the presence of impurities. Therefore, it would be advantageous to have a process of preparing a carbon membrane having high fouling resistance and having high selectivity and having good performance in high pressures. The process of the instant invention meets these needs.